This invention is related to arc tubes used in metal halide discharge lamps. More particularly, this invention is related to cylindrical quartz arc tubes for metal halide lamps.
Low wattage metal halide lamps (35-150 Watts) are potential candidates to replace incandescent lamps in general lighting and commercial display applications because they offer higher efficacy and longer life. However, compared to incandescent lamps, low wattage metal halide lamps frequently exhibit inferior color rendering and variable (lamp-to-lamp) color consistency. Therefore, alternative design approaches are being sought to address the color deficiencies, without sacrificing the high efficacy and long life.
In commercial metal halide lamps, the arc tube is made from a section of quartz tubing. Each end of the quartz tube is pinched between a pair of opposed jaws to form a gas-tight seal about an electrode assembly while the quartz is in a heat-softened condition. As a result of this pinch-seal process, the ends become somewhat deformed and rounded between the cylindrical main body of the arc tube and the flattened press seal area. The curved shape of these end wells may vary with the diameter and wall thickness of the original quartz tubing, the heat concentration during processing, and the pressure of the enclosed inert gas during pressing.
The photometric performance parameters of metal halide lamps are dependent on the partial pressures of the enclosed metal halide salts. Their vapor pressures are primarily controlled by the arc tube wall temperature in the region where the metal halide vapors condense. This zone is usually located in the lowest portion of the arc tube due to gravity and internal gas convection flow. The temperature of this so-called xe2x80x9ccold zonexe2x80x9d should be high enough to provide sufficient evaporation of the radiating metal halide species. However, the temperature cannot be too high otherwise the long life of the arc tube will be compromised due to chemical reactions with the wall or devitrification of the quartz. Therefore, a nearly uniform wall temperature distribution (not exceeding about 900xc2x0 C. for quartz) is desirable for a useful life of more than about 6000 hours. The 900xc2x0 C. wall temperature is high enough for evaporating many metal halide salts and low enough to realize a useful life of the arc tube. In the case of lamps that use quartz arc tubes, lamp life typically is reduced by a factor of two for every 50xc2x0 C. increase over 900xc2x0 C.
One of the known means for realizing a more uniform wall temperature distribution is applying a heat-conserving coating, such as zirconium oxide, to the outside surface of the end wells of the arc tube. Most conventional metal halide lamps utilize this heat-conserving coating on one or both ends of the arc tube. Apart from being an additional cost component, the coating is itself a significant source of variability in the photometric performance of such lamps because of intrinsic lamp-to-lamp variation in coating height, adhesion properties, and its tendency to discolor.
A more effective but more costly way of obtaining a nearly uniform wall temperature distribution is to form discharge vessels in elliptical or pear-shaped bodies for vertically operated lamps or arched tubes for horizontal operation. However, this method does not generally provide for universal operation of the lamp (i.e., a lamp oriented arbitrarily with respect to gravity), and requires time consuming glass-working steps that are not needed for straight tubular body arc tubes.
High arc loading (W/cm) and wall loading (W/cm2) are critical for improved performance in low wattage metal halide lamps. Typically, for 35W to 150W quartz-body arc tubes of conventional design, average electrical wall loading does not exceed 20 W/cm2 (or 100 W/cm arc loading) in order to obtain an operating life of greater than about 6000 hours. These empirically determined limits result from the fact that at elevated loading the temperatures on the arc tube wall become too high for quartz to survive through the desired life. To remain within these loading limits, lamp designers have adjusted the arc chamber size and shape, specifically, the electrode insertion length, lamp cavity length, and lamp diameter in elliptical or ellipsoidal design arc tubes. Additional control of temperature distributions and levels in metal halide lamps has been exercised by changes in the arc tube fill chemistry.
Cylindrical quartz arc tubes with conservatively low wall loadings (10-13 W/cm2) were rejected in the early days (1960""s) of metal halide lamp development because they did not provide adequate efficiency in low wattage lamps. Nearly symmetric longitudinal, outer surface temperature profiles have been achieved with ceramic arc tubes having a right circular cylindrical shape, e.g., U.S. Pat. Nos. 5,424,609 and 5,751,111. However, the operating temperatures of ceramic arc tubes is typically above 975xc2x0 C. which far exceeds the 900xc2x0 C. limit for quartz arc tubes.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a quartz arc tube for a metal halide lamp which can be operated at a high average wall loading without exceeding a maximum surface temperature of the discharge chamber of about 900xc2x0 C.
It is yet another object of the invention to provide a quartz arc tube for a metal halide lamp which has a nearly symmetric longitudinal surface temperature profile when operating at a steady-state thermal condition.
It is still another object of the invention to provide a method for making quartz arc tubes for a metal halide lamps having these desirable properties.
In accordance with one object of the invention, there is provided a quartz arc tube for a metal halide lamp comprising a quartz body enclosing a discharge chamber having a metal halide fill, the discharge chamber having substantially the shape of a right circular cylinder and containing opposing electrodes, the discharge chamber having a nearly symmetric longitudinal surface temperature profile when operating in a steady-state thermal condition wherein the difference between the maximum and minimum temperatures of the profile is less than about 30xc2x0 C. and the maximum temperature of the profile is less than about 900xc2x0 C.
In accordance with another object of the invention, there is provided a quartz arc tube for a metal halide lamp comprising a quartz body enclosing a discharge chamber having a metal halide fill, the discharge chamber having substantially the shape of a right circular cylinder and containing opposing electrodes, the opposing electrodes being disposed at each end of the discharge chamber and coaxial with the axis of the chamber, the distance between the opposing electrodes defining an arc length, the inner diameter of the discharge chamber in centimeters being approximately equal to [(1+P/50)1/2xe2x88x921], where P is the input power in watts, and wherein the ratio of the arc length to the inner diameter is about one.
In accordance with yet another object of the invention, there is provided a method of making a quartz arc tube for a metal halide lamp, the quartz arc tube having a quartz body enclosing a discharge chamber having a metal halide fill, the discharge chamber having substantially the shape of a right circular cylinder and containing opposing electrodes, the opposing electrodes being disposed at each end of the discharge chamber and coaxial with the axis of the chamber, the distance between the opposing electrodes defining an arc length, the discharge chamber having a pierce point where each corresponding electrode enters the discharge chamber, the distance between the pierce point and the corresponding electrode end within the discharge chamber defining an electrode insertion length, the arc tube when operating in a steady-state thermal condition having a longitudinal surface temperature profile, the method comprising the steps of:
a) selecting an arc length and an inner diameter for the discharge chamber wherein the inner diameter in centimeters is greater than [(1+P/50)1/2xe2x88x921], where P is the input power in watts, and wherein the ratio of the arc length to the inner diameter is about one;
b) forming the arc tube;
c) operating the arc tube at a predetermined average wall loading to obtain a steady-state thermal condition;
d) measuring a longitudinal surface temperature profile of the discharge chamber to obtain a maximum temperature and minimum temperature;
e) repeating steps b) to d) while incrementally decreasing the inner diameter of the discharge chamber with each iteration until the maximum temperature of the longitudinal surface temperature profile is midway between the ends of the discharge chamber; and
f) repeating steps b) to d) while incrementally varying the electrode insertion length with each iteration until the difference between the minimum temperature and the maximum temperature of the profile is minimized without causing the maximum temperature to exceed about 900xc2x0 C.